Electroless metal deposition is a well-known process for depositing metallic layers on substrate surfaces. Electroless plating of a dielectric surface requires the prior application of a catalyst. The most commonly used method of catalyzing or activating dielectrics, such as non-conductive sections of laminated substrates used in the manufacture of printed circuit boards, is to treat the substrate with an aqueous tin/palladium colloid in an acidic chloride medium. The structure of the colloid has been extensively studied. In general, the colloid includes a palladium metal core surrounded by a stabilizing layer of tin(II) ions, essentially a shell of SnCl3− complexes which act as surface stabilizing groups to avoid agglomeration of the colloids in suspension.
In the activation process the tin/palladium colloid catalyst is adsorbed onto a dielectric substrate, such as epoxy or polyimide containing substrate, to activate electroless metal deposition. Theoretically the catalyst functions as a carrier in the path of electron transfer from reducing agents to metal ions in the electroless metal plating bath. Although performance of electroless plating is influenced by many factors, such as additive composition of the plating solution, the activation step is key for controlling the rate and mechanism of electroless plating.
In recent years, along with the reduction in size and desired increase in the performance of electronic devices, the demand for defect free electronic circuits in the electronic packaging industry has become higher. Although the tin/palladium colloid has been commercially used as an activator for electroless metal plating for decades and has given acceptable service, it has many disadvantages which are becoming more pronounced as the demand for higher quality electronic devices increases. The stability of the tin/palladium colloid is a major concern. As mentioned above the tin/palladium colloid is stabilized by a layer of tin(II) ions and its counter anions can prevent palladium from agglomerating. The catalyst is sensitive to air and readily oxidizes to tin(IV), thus the colloid cannot maintain its colloidal structure. This oxidation is further promoted by increase in temperature and agitation during electroless plating. If the concentration of tin(II) falls to critical levels, such as close to zero, palladium metal particles grow in size, agglomerate and precipitate, thus becoming catalytically inactive. As a result there is an increase in demand for a more stable catalyst. In addition the high and fluctuating cost of palladium has encouraged the industry to search for a less costly metal.
Considerable efforts have been made to find new and improved catalysts. Because of the high cost of palladium, much effort has been directed toward development of palladium free catalysts, such as colloidal silver catalysts. Another direction that research has taken is towards a tin free palladium catalyst since stannous chloride is costly and the oxidized tin requires a separate acceleration step. In addition to adding a further step in the overall electroless process, materials used in the acceleration step often strip the catalyst from the substrate to be plated leaving undesirable voids in the plating. This is especially common on glass fiber substrates which are typically used in printed circuit board manufacturing. However, such tin free catalysts have shown to be insufficiently active and reliable for through-hole plating in printed circuit board manufacture. Further, such catalysts typically become progressively less active upon storage, thus rendering such catalysts unreliable and impractical for commercial use.
Alternative stabilizing moieties for tin complexes, such as polyvinylpyrrolidone (PVP) and dendrimers, have been investigated. Stable and uniform PVP protected nanoparticles have been reported by various research groups in the literature. Other metal colloids, such as silver/palladium and copper/palladium in which palladium is partially replaced by less expensive metals have also been reported in the literature; however, to date there has been no commercially acceptable alternative for the tin/palladium colloidal catalyst. Accordingly, there is still a need for a stable and reliable electroless metal plating catalyst.